1. Field of the Invention
The present invention relates to television (TV) signal transmission systems in general and in particular to an apparatus for transmitting scrambled video digital signals in a cable TV (CATV) system.
2. Description of the Prior Art
A typical CATV system comprises a headend station and, depending on its size, one or more hub stations for transmitting TV signals to the homes, offices, and the like, of individual subscribers.
A headend station is defined as a source of TV signals. The signals are typically transmitted from the headend station to one or more hub stations via fiber optic, coaxial and/or microwave transmission lines. The signals are transmitted from a hub station, usually by means of coaxial transmission lines, to the TV sets of individual subscribers and, in the case of large CATV systems, to other hub stations for distribution to its subscribers.
Conventional TV sets are designed to receive amplitude modulated (AM) video signals. The transmission of AM signals, however, suffers from poor performance due to decreasing signal-to-noise ratios which can become unacceptable over long transmission lines, e.g. longer than 5-10 kilometers.
To compensate for the signal-to-noise problem where long transmission lines exist between a headend station and a hub, a common practice has been to transmit FM video signals between the two. As is well known, such signals have a very high signal-to-noise ratio. When received in the hub, the signals are demodulated and then amplitude modulated for transmission to the TV sets connected thereto.
The scrambling of video signals is used in CATV systems to prevent the unauthorized viewing of certain TV programs or channels. While various methods may be used for scrambling a video signal, the most common method currently in use involves producing a radio frequency (r.f.) video signal by modulating the amplitude of a carrier signal by a baseband video signal which comprises video sync pulses, and then suppressing the video sync pulse in the amplitude modulated baseband r.f. video signal. In the TV set, sync circuits which are responsive to the sync pulses are used for synchronizing the TV to the transmitted video. If the sync pulses are suppressed, a descrambler is required in order to restore the pulse to the AM signal.
In systems providing scrambled video signals which must be transmitted over long distances, e.g. longer than 5-10 kilometers, FM signal processing has been used in an attempt to compensate for the reduced signal-to-noise ratios encountered. In practice, however, since the resulting FM modulation typically produces a modulation within a modulation due to the use of amplitude modulation in the scrambling process, the use of FM to compensate for the adverse effects on the signal-to-noise ratio resulting from the AM modulation is not found to be adequate.
In applicant's copending application Ser. No. 07/447,363, entitled Scrambled Frequency Modulated Video Signal Transmission System, assigned to the assignee of the present application, there is disclosed a system for transmitting scrambled FM modulated video signals from which the AM modulation has been removed before their transmission. This system seeks to eliminate the reduction in the signal-to-noise ratio associated with the transmission of AM modulated signals, i.e. signals comprising a modulation within a modulation, while preserving the advantages associated with transmitting purely FM modulated signals. While providing certain advantages, applicant's above-described system is relatively complex and therefore relatively costly to produce and maintain.